Natural killer (NK) cells play a critical role in the early innate immune response to Chlamydophila abortus infection in mice.

Chlamydophila abortus, the aetiological agent of ovine enzootic abortion, induces a strong inflammatory reaction that leads to the T helper cell (Th1) specific immune response necessary for the clearance of infection. Because the role of natural killer (NK) cells during the first stages of this response has received little attention, this study focused on determining the function of these cells in a mouse model of infection. The location of NK cells in the liver and spleen of infected mice was examined immunohistochemically with an anti-Ly49G monoclonal antibody. The number of NK cells increased during the infection both in spleen and liver. In subsequent experiments, an anti-asialo GM1 polyclonal antibody was injected to deplete the NK cells. NK-depleted mice showed a substantial increase in their susceptibility to C. abortus infection, with high mortality rates and an increased burden of bacteria in the liver. Histopathological studies showed that inflammatory foci, composed mainly of neutrophils, were greater in size and number in depleted mice, while numerous chlamydial inclusions were associated with the foci. Serum concentrations of IFN-gamma, a key cytokine in the control of C. abortus infection, were substantially reduced in the NK-depleted mice. To establish the relationship between NK cells and other components of the innate immune response, neutrophils were depleted with the RB6-8C5 antibody. These cells were shown to be crucial in the recruitment of NK cells to the inflammatory foci.